Cluster: new command flag forcing implicit ASKING.

Also using this new flag the RESTORE-ASKING command was implemented that
will be used by MIGRATE.
Use RESTORE-ASKING for MIGRATE when in cluster mode.
Cluster: always use cluster(Add|Del)Slot to modify the cluster slots table.
Cluster: clusterAddSlot() was not doing what stated in the comment.
redis-trib: some refactoring and skeleton of the "fix" command.
redis-trib: move instance vars in the right class.
Aesthetic change to make a line more 80-cols friendly.
Call clusterUpdateState() after CLUSTER SETSLOT too.
Cluster: more correct update of slots state when PONG is received.
redis-trib: fixed typo in method name.
redis-trib: ability to fix uncovered slots for the trivial case.